1. Field of the Invention
The present invention relates to a testing of a self-commutative electric power conversion device for converting DC electric power to AC electric power, and more particularly, to a method and an apparatus for the verifying of a the voltage stress and a current stress in the self-commutative electric power conversion device.
2. Description of the Background Art
In a conventional method of testing the voltage stress and the current stress in a main circuit of a self-commutative electric power conversion device, a load equivalent to the rated load of the device is connected on its AC output side, while a power source having a capacity to supply a sufficient output voltage on the load is provided on its DC input side, and then the load test is carried out by operating a control circuit of the self-commutative electric power conversion device or an equivalent testing circuit.
Now, in such a conventional testing method, the voltage stress and the current stress of the main circuit can be verified only for the voltages below the rated voltage and the currents below the overloading rated current. However, the protection levels for the excessive voltage and the excessive current in the device are usually set at values higher than the rated voltage and the rated current of the device. As a consequence, it has been impossible in a conventional testing method to verify whether the voltage stress and the current stress exerted on the device are below the tolerable levels In a case the protection against the excessive voltage is activated or a case the cut off of the excessive current is made.
On the other hand, in recent years, the self-commutative electric power conversion device with a very large capacity has been demanded by such a practical application as the power source for a large size linear motor car, the large scale frequency conversion, and the large scale reactive power compensation, so that the capacities of the power source and tile load required in testing such a very large capacity self-commutative electric power conversion device by the above described conventional testing method also become very large, and consequently the testing of such a very large capacity self-commutative electric power conversion device in the factory requires tile enormous cost and space for the testing facility.
Furthermore, the self-commutative electric power conversion device generally uses PWM (Pulse Width Modulation) controlling. However, a very large capacity is usually realized in the self-commutative electric power conversion device by connecting a large number or circuit elements in series, and when such a very large capacity self-commutative electric power conversion device is operated under the PWM controlling, it is very difficult to measure the maximum voltage stress and the maximum current stress in the main circuit of the device by using a usual measurement equipment, because it is difficult to make the simultaneous measurements of the voltage and the current in such a very large capacity self-commutative electric power conversion device due to the presence of a series connection of a large number of circuit elements, and consequently it is difficult to identify the proper timing for the occurrence of the maximum voltage stress and the maximum current stress.